Use of Aztreonam Plus Ceftazidime/Avibactam Combination for In vitro Susceptibility Testing of CRO Isolates in a Tertiary Care Hospital – Our experience

Background: Infections caused by metallo-beta-lactamases (MBLs) producing Gram-negative bacteria (mainly Enterobacterales) pose a great challenge for clinicians in treating these multidrug-resistant infections. In recent times, combination antibiotic therapy with ceftazidime–avibactam (CAZ-AVI) with aztreonam (ATM) has been researched by a number of laboratories worldwide and has gained much clinical attention. This study evaluated a practical laboratory method of testing for clinically significant in vitro synergy between CAZ/AVI + ATM in New Delhi MBLs (NDM) producing Gram-negative organisms. Materials and Methods: A total of 100 isolates of carbapenem-resistant Enterobacteriaceae ( Klebsiella pneumoniae , Escherichia coli , Pseudomonas aeruginosa , and Enterobacter cloacae ) were tested for synergy between CAZ/AVI + ATM using E-strip of CAZ/AVI and disc of ATM. The minimum inhibitory concentration value of CAZ/AVI was measured and the zone of inhibition was observed when ATM was placed parallel to the E-strip on a lawn culture of an isolate. Simultaneously, all isolates were tested for NDM and OXA-48 by Xpert Carba-R. Results: Of 100 isolates, 63/100 (63%) were harboring OXA-48 beta-lactamase, 54/100 (54%) were harboring NDM beta-lactamase and 18/100 (18%) were harboring both (OXA-48 and NDM). In vitro synergy between CAZ/AVI + ATM was noted in 99 isolates (99%). Conclusion: A significant synergy was demonstrated in vitro with CAZ/AVI + ATM combination therapy and seems to be a promising treatment strategy for infections caused by Enterobacterales harboring NDM and OXA-48 beta-lactamases.